The formation of an active device structure or mesa requires effective masking and etching procedures. In order to have an optimium structure for n-channel transistors, it is important to provide a pad oxide to cover the active area silicon during the implantation of boron. If there is no pad oxide covering the silicon during implantation, the implantation process will damage the surface of the silicon causing future topographic discontinuities.
Additionally, impurities have been found on the silicon surface after a mesa has been formed. These impurities or spikes are formed as a result of polymers that are a biproduct of the etching processes used to form the mesa. At best the spikes are merely obstructions to subsequent structure formation but if the spikes are too close to the mesa, an electrical short may be created.
In a prior method disclosed in U.S. Pat. No. 4,753,896, June 28, 1988, to Matloubian, assigned to Texas Instruments, Inc., which is incorporated herein by reference, an oxide/nitride/oxide stack is used to mask the active area. The etchants used in the process are not sufficiently selective between nitride and oxide, which may cause removal of the pad oxide, exposing the active silicon surface. The exposed silicon surface may then be damaged by the implantation of boron.
Additionally in the prior art, the etchants tend to produce polymers over the nitride. Once the polymers form, they serve as a mask to prevent continued etching of the nitride. Therefore, a mask of nitride over oxide remains in some locations, serving as a starting point for a silicon pillar or spike. Thus, there is a need for an improved masking scheme to form a silicon dioxide mesa without damaging the active area silicon and without creating spikes thereon.